Method for operating fuel cells with passive reactant supply

ABSTRACT

A method for operating a passive, air-breathing fuel cell system is described. In one embodiment, the system comprises one or more fuel cells, and a closed fuel plenum connected to a fuel supply. In some embodiments of the method, the fuel cell cathodes are exposed to ambient air, and the fuel is supplied to the anodes via the fuel plenum at a pressure greater than that of the ambient air.

PRIORITY OF INVENTION

This non-provisional application claims the benefit of priority under 35U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No.60/743,173, filed Jan. 25, 2006, which is herein incorporated byreference.

FIELD

The present invention relates to fuel cells and, more particularly, tomethods of operating passive, air-breathing fuel cells having closedfuel supply systems. Embodiments of the method can be used to extendoperating time and achieve high fuel utilization.

COPYRIGHT

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent files or records, but otherwise reserves all copyrightrights whatsoever. The following notice applies to the software and dataas described below and in the drawings that form a part of thisdocument: Copyright 2005, Angstrom Power Inc. All Rights Reserved.

BACKGROUND

Electrochemical fuel cells convert a fuel and an oxidant to electricity.Solid polymer electrochemical fuel cells generally employ an ionexchange membrane or some other kind of solid polymer electrolytedisposed between two electrodes, an anode and a cathode, each comprisinga layer of catalyst to induce the desired electrochemical reaction. Anembodiment of a conventional hydrogen fuel cell system is shownschematically at 10 in FIG. 1. It includes an anode 12 and a hydrogengas inlet 14, and a cathode 18 and an air inlet 20. Hydrogen gas entersthe fuel cell at the inlet 14 and is oxidized at anode 12 to formprotons 16 and electrons 17. Oxygen, often from air, is reduced atcathode 18 to form water 22. The fuel cell system also includes a protonexchange membrane 24 for passage of protons from the anode 12 to thecathode 18. In addition to conducting hydrogen ions, the membrane 24separates the hydrogen fuel stream from the oxidant stream. Aconventional fuel cell also includes outlets 25 and 26 for oxidant andfuel, respectively.

In many conventional fuel cells, electrically conductive reactant flowfield plates are used to direct pressurized reactant streams, which maybe pressurized, to flow across the anode and cathode between thereactant stream inlet and outlet. Typically such reactant flow fieldplates have at least one flow passage or channel formed in one or bothfaces. The fluid flow field plates act as current collectors, providesupport for the electrodes, provide access channels or passages for thefuel and oxidant to the respective anode and cathode surfaces, andprovide passages for the removal of reaction products, such as water,formed during operation of the cell.

Fuel cell performance can suffer significantly if there is not asufficient supply of reactant to the entire electrode. Therefore, it hasbeen a common practice in conventional fuel cells to provide excessreactants to the fuel cell in order to assure adequate supply at theelectrode. In the case of the anode electrode, this generally wastesvaluable fuel—reducing the fuel utilization, which is the ratio of thequantity of fuel supplied to the quantity of fuel actually consumed toproduce electrical power. Ideally all of the fuel supplied to the fuelcell is used to produce power (a fuel utilization of 1 or 100%).

Some fuel cells are designed to operate in a closed mode on one or bothof the reactant sides in an attempt to try to increase the reactantutilization. In these situations the reactant used on the closed side isgenerally substantially pure. Nonetheless, one of the problemsassociated with such systems is the accumulation of non-reactivecomponents that tend to build up on the anode and dilute the local fuelconcentration. If the fuel supply needed to support the power demand isnot available (even locally within a particular fuel cell in thesystem), the fuel cell system may experience global or localized fuelstarvation. Fuel starvation can cause permanent, irrecoverable, materialdamage to the fuel cells resulting in lower performance or eventualfailure of the system.

There are various sources of the non-reactive components that tend toaccumulate at the anode in a closed fuel system. One is impurities inthe fuel stream itself—even if the fuel is substantially pure with avery low concentration of other components, these will tend to build upover time in a closed system. Also water produced at the cathode andnitrogen from the air (in air breathing configurations) will tend tocross the electrolyte and accumulate at the anode

A typical solution is the inclusion of a purge valve (which is normallyclosed in closed system operation) somewhere in the fuel passage forperiodic venting of accumulations of non-reactive components, which canbuild up at the anode in closed system operation. In conventional fuelcell purge systems the purge valve is opened from time to time, forexample, manually or at regular fixed time intervals, or in response tosome monitored parameter. Alternatively, a continuous small vent ofreactant may be used to prevent the accumulation of non-reactivecomponents. The reactant flow path through the fuel cell system can beconfigured so that non-reactive components tend to accumulate first injust one or a few fuel cells of the fuel cell assembly, rather than inthe outlet region of each cell in the assembly.

Such systems are not truly dead-ended, and although purging or acontinuous vent can improve performance of fuel cells having closed fuelsupply systems, it wastes valuable fuel—thereby reducing the fuelutilization. It also increases the parasitic load on the system and thecomplexity if purging equipment is required. Furthermore, the release ofhydrogen into the ambient environment may be undesirable.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view of a conventional fuel cell.

FIG. 2 is a graphical view showing fuel cell voltage against operatingtime for a passive, air-breathing 10-cell fuel cell system operatedunder a variety of conditions.

FIG. 3 is a graphical view showing fuel cell voltage against operatingtime volts for a passive, air-breathing 10-cell fuel cell systemoperated dead-ended on hydrogen with an approximately 24 psig pressuredifferential from anode to cathode.

FIG. 4 is a graphical view showing fuel cell voltage against operatingtime for extended dead-ended operation of a passive, air-breathingplanar fuel cell array with an approximately 5 psig pressuredifferential from anode to cathode.

FIG. 5 is an exploded perspective view of an embodiment of a fuel cellsystem of the invention.

DETAILED DESCRIPTION

The following detailed description includes references to theaccompanying drawings, which form a part of the detailed description.The drawings show, by way of illustration, specific embodiments in whichthe invention may be practiced. These embodiments, which are alsoreferred to herein as “examples,” are described in enough detail toenable those skilled in the art to practice the invention. Theembodiments may be combined, other embodiments may be utilized, orstructural, and logical changes may be made without departing from thescope of the present invention. The following detailed description is,therefore, not to be taken in a limiting sense, and the scope of thepresent invention is defined by the appended claims and theirequivalents.

In this document, the terms “a” or “an” are used to include one or morethan one and the term “or” is used to refer to a nonexclusive or unlessotherwise indicated. In addition, it is to be understood that thephraseology or terminology employed herein, and not otherwise defined,is for the purpose of description only and not of limitation.Furthermore, all publications, patents, and patent documents referred toin this document are incorporated by reference herein in their entirety,as though individually incorporated by reference. In the event ofinconsistent usages between this document and those documents soincorporated by reference, the usage in the incorporated referenceshould be considered supplementary to that of this document; forirreconcilable inconsistencies, the usage in this document controls.

Although detailed embodiments of the invention are disclosed herein, itis to be understood that the disclosed embodiments are merely exemplaryof the invention that may be embodied in various and alternative forms.Specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a basis for teaching oneskilled in the art to variously employ the fuel cell operationembodiments. Throughout the drawings, like elements are given likenumerals. Embodiments of the method for fuel cell operation describedherein apply to fuel cell power generation in general, includingtransportation applications, portable power sources, home and commercialpower generation, large power generation, small system power generationand to any other application that would benefit from the use of such asystem.

The invention embodiments described herein relate to a method ofoperating a passive, air-breathing fuel cell that has a closed fuelsupply.

As used herein, “passive” refers to the flow of a reactant utilizing noexternal mechanical power. For example, the flow of a reactant may becaused by diffusion or a difference in pressure gradient. Under passiveoperation in a fuel cell system, the pressure of a reactant may beregulated, modulated, or varied, for example.

As used herein, “dead ended” refers to a fuel cell or fuel cell systemin which a fuel is not recirculated through the fuel cell orexhausted/released/expelled from the fuel supply. For example, any fuelthat passes from a fuel source to one or more fuel cells is consumed bythe fuel cell reaction. For dead ended operation, the fuel cell or fuelcell system includes a closed plenum, for example. Dead ended fuel cellsystems include a fuel outlet which is closed for some embodiments andfor other embodiments, dead ended fuel cell systems do not include afuel outlet.

As used herein, “pressure” refers to a force applied uniformly over asurface and may be measured as force per unit of area. For example, apressure of a reactant or fuel may be regulated or varied with use in afuel cell system. Pressure as used herein, includes both absolutepressure measurement and relative pressure measurement.

As used herein, “purge” or “purging” refers to venting, releasing, orremoving of a substance or substances. For example, for someembodiments, such substances may include accumulations of non-reactivecomponents or contaminants. For example, non-reactive components maybuild up at the anode in closed fuel cell system and may be removed bypurging, such as opening of a valve.

As used herein, the term “fuel supply” refers to any structure orassembly that stores a fuel. One example of a fuel is hydrogen. In afuel supply, the fuel may be stored using a variety of mechanisms. Forexample, in a hydrogen fuel supply, hydrogen may be stored as a metalhydride, composite metal hydride, carbon-graphite nanofibers, compressedhydrogen gas, chemical hydrides or combinations of these materials. Forsome embodiments, a fuel supply also includes a fuel storage materialand components in addition to the fuel storage material. For someembodiments, the fuel supply is internal, such as a fuel reservoir. Forother embodiments, the fuel supply is external or removable, such as afuel cartridge. For other embodiments, the fuel supply is a combinationof internal and external components, such as a cartridge that fills areservoir which supplies fuel to the fuel to the anodes of the fuel cellsystem, optionally via a fuel plenum.

As used herein, the terms “fuel plenum,” “fuel enclosure” and “fuelchamber refer to structures that contain fuel, which may be in fluidcontact with the anodes of a fuel cell. Fuel plenums, fuel enclosures,and fuel chambers include embodiments which are flexible, embodimentswhich are integrally formed in the fuel cell system, and embodimentswhich may be a variety of shapes and sizes.

In most conventional fuel cells there is typically forced flow of fuelto the anode, although in some cases the fuel is supplied from apressurized source. Typically fuel cell systems also incorporate somekind of active flow control which adjusts the rate of supply of one orboth reactants in response to the fuel cell power output demand or someother parameter. Often a rotameter or mass flow controller is used.

In a passive, air-breathing fuel cell, the cathode is merely exposed toambient air. When the fuel cell is operating, the cathode consumesoxygen from the surrounding air to support the fuel cell reaction. Airis thus supplied to the cathode by diffusion. There is no active flowcontrol of oxidant to the cathode, and there is no oxidant inlet oroutlet per se. For some embodiments, fuel cell assemblies with passivereactant supply include varied, regulated, or modulated pressureoperation.

The closed fuel enclosure means that the fuel supply to the fuel cell isdead-ended. Fuel fluidly contacts the anodes of the fuel cell assemblyand is consumed through a reduction reaction. As fuel is consumed, itmay flow from the fuel supply into a fuel plenum, for example by forcedconvection if the fuel is pressurized. Depending upon the configurationof the fuel flow path there may be fuel flow from one cell to the nextas the reaction proceeds, but the overall assembly has a closed fuelenclosure with no outlet and no venting or bleeding of fuel on the anodeside; however, it is recognized that in certain circumstances there maybe small amounts of fuel lost via diffusion through the electrolyte tothe cathode.

In one embodiment, a fuel cell system, shown in an exploded perspectiveview at 100 in FIG. 5 includes, among other things, at least one fuelcell layer 102 that includes an anode 108 and a cathode 107 with anion-conductive electrolyte 109 disposed there between and a fuel supply(e.g. a fuel cartridge or internal fuel reservoir) 104, In variousexamples, fuel supply 104 optionally comprises a refueling port 112and/or a pressure regulator 110. Refueling port 112 is a pressureactivated valve that allows a flow of fluid, for example, fluid fuel,into the fuel supply 104.

A fuel enclosure, or fuel plenum, (not shown) can be created bypositioning fuel cell layer 102 adjacent to at least one surface of thefuel reservoir 104. A perimeter of the fuel supply 104 surface incontact with fuel cell layer 102 may be sealed by a seal member 126,such as a (compressive or elastic) gasket or an adhesive, therebyforming a closed fuel enclosure (not shown). In the exemplaryembodiment, pressure regulator 110 fluidically connects the fuel supply104 to the fuel enclosure, or plenum (not shown).

An embodiment of the method for operating such a fuel cell systemincludes exposing the cathode(s) 107 to ambient air, and supplying afuel stream to the anode(s) 108 via the fuel plenum (not shown) at apressure greater than that of the ambient air.

The use of a positive pressure differential from the anode 108 to thecathode 107 has been found to improve the performance and/or extend theoperating time and/or allow achievement of high fuel utilization in apassive, air-breathing fuel cell system. With use of a positive pressuredifferential, fuel utilizations greater than 75%, or even greater than90% may be achievable.

It is believed that a higher fuel pressure on the anode impedes themigration of nitrogen from the air on the cathode side. Nitrogenaccumulation in the closed fuel enclosure would eventually result in atleast localized fuel starvation with a drop in fuel cell performance,and potentially eventual damage to the fuel cell itself. However, if thepressure differential is too large (fuel pressure too great) too muchhydrogen crossover from the anode to the cathode will occur. This wastesfuel (reducing the fuel utilization), and can impede the oxidationreaction at the cathode.

In addition, the use of pressure differential from anode 108 to cathode107 may allow for modification of water management behavior of the cell.This can have significant impact on cell operation as the presence ofwater can affect everything from proton conduction in the electrolyte toreactant gas access in the electrodes.

In some embodiments of the method the fuel is substantially purehydrogen. For example, the hydrogen may be supplied from a compressedhydrogen source, a hydrogen storage material such as a metal hydride, acomposite metal hydride, carbon-graphite nanofibers, or a chemicalhydride hydrogen source. There are several metal hydrides that arepossible for use as a hydrogen storage material, which are generallygrouped by their crystal structure (i.e. AB₅, AB₂, AB BCC). The hydridecan be a metal or metal alloy. Examples of hydrides include, but are notlimited to: LaNi₅, FeTi, a mischmetal hydride (a mixture of metal or anore, such as MmNi₅), vanadium hydride, magnesium hydride, intermetallichydride, solid solution hydride, multiphase hydride, composite hydride,alloys thereof, or solid solutions thereof. Examples of chemical hydridehydrogen sources include, but are not limited to: sodium borohydride,sodium alanate, and lithium alanate.

In some embodiments the fuel is supplied to the fuel enclosure via apressure regulator, for example, as shown at 110 in FIG. 5. The fuel maybe supplied at constant pressure or variable pressure. The pressure atwhich the fuel is supplied may be modulated in response to an aspect ofsystem performance, such as power demand of the fuel cell or fuel celllayer, for example. The fuel is supplied without active flow control(e.g. without using mass flow meter or rotameter); in some embodimentsof the method, the pressure at which the fuel is supplied to the anodecan be independent of the power demanded from the fuel cell or fuel celllayer, for example, as shown at 102 in FIG. 5. In some embodiments thepressure of fuel supplied to the fuel enclosure (not shown) isunregulated. For example, the fuel enclosure may be fluidly connected toa metal hydride hydrogen storage system so that it accepts hydrogen fromthe metal hydride at whatever pressure the hydrogen is discharged fromthe metal hydride. The method embodiments described herein may beimplemented in a wide variety of fuel cell architectures that can beconfigured to be passive, air-breathing fuel cells. For example,embodiments of the method can be used with fuel cell assemblies of thetypes described in commonly owned U.S. patent application Ser. Nos.10/887,519 entitled COMPACT CHEMICAL REACTOR; 10/818,610 entitledCOMPACT CHEMICAL REACTOR WITH REACTOR FRAME; 10/818,611 entitled FUELCELL LAYER; 10/818,843 entitled FUEL CELL LAYER WITH REACTOR FRAME; and11/047,557 entitled ELECTROCHEMICAL FUEL CELLS FORMED ON PLEATEDSUBSTRATES all incorporated herein by reference. As another example,embodiments of the method can be used with fuel cell assemblies of thetype described in commonly owned U.S. patent application Ser. No.11/047,560 entitled ELECTROCHEMICAL FUEL CELLS HAVING CURRENT-CARRYINGSTRUCTURES UNDERLYING REACTION LAYERS (also incorporated herein byreference) that include planar fuel cell arrays.

Fuel cells within the assembly can be electrically connected in parallelor in series, or in sub-groups comprising combinations of the two.Implementation of the present method is essentially independent of theway in which the fuel cells in the assembly are electrically connectedto one another

The closed fuel enclosure can be configured in a variety of ways. Forexample it may be configured so that the fuel is supplied to each of aplurality of anodes in parallel, or so that the fuel is supplied to someor all of anodes in series, or in some other configuration. Again,implementation of the present method embodiments is independent of theway in which the anodes in the assembly are fluidly connected to oneanother, although it may be optimized for a particular design.

It is not a requirement for the fuel cell assemblies to incorporatediscrete flow channels for directing reactants across the surface of theelectrodes, as in conventional fuel cells.

In some embodiments, it is contemplated that the fuel supply 104 isdirectly coupled to the fuel cell assembly so that the fuel isintegrally contained between anodes and fuel supply in such a way that afuel plenum is no longer an explicit component of the fuel cell system,but instead may be considered to be implicitly created throughintegration of other components of the system. In some embodiments, thefuel plenum is directly integrated into the fuel supply, such that thefuel supply and the fuel plenum essentially become one entity.

Exposed cathodes may require protection from a variety of hazards. Suchhazards could include, but are not limited to, physical damage such asabrasion or puncture, excess drying, excess moisture and airbornecontaminants such as SO₂, CO, and CO₂, that can be detrimental to theperformance of the catalyst and/or fuel cell. Accordingly, the fuel cellsystem may include mechanisms for protecting the cathodes. In addition,such mechanisms may also be used to affect, modify, and/or control thewater management aspects of the system. Examples of such mechanismsinclude, but are not limited to:

-   1. A carbon layer deposited within the gas diffusion layer that is    activated to absorb contaminants.-   2. A hydrophobic layer deposited on the surface of the fuel cell    that renders the cathode water repellent.-   3. A porous cover over the fuel cell comprised of:    -   i. a porous, hydrophobic Teflon® sheet    -   ii. a porous activated carbon filter-   4. A screen or mesh cover.

These mechanisms for protecting the cathodes may be used independentlyor in collaboration with one another. It is understood that thesemechanisms are only examples of methods to protect the cathodes, not anexhaustive list.

In some embodiments, the fuel cell system includes a fuel enclosureinlet and a fuel enclosure outlet, which is plugged. For someembodiments, the fuel cell system does not include a fuel enclosureoutlet at all. The fuel cell system may include a cathode that isexposed to, or in fluid contact with, the surrounding air. The fuel cellsystem also includes an electrolyte disposed between an anode and acathode. For some embodiments, the electrolyte comprises an ion exchangemembrane, or ion conductive electrolyte.

If present, the fuel enclosure outlet is plugged in order to preventhydrogen from venting from the fuel cell system, effectively dead-endingthe fuel enclosure. Method embodiments described herein also includeoperating the fuel cell system at a fuel pressure that is effective forreducing nitrogen diffusion across the electrolyte.

The method embodiments described herein improve fuel cell efficiency andperformance by identifying an effective fuel pressure and applying thatfuel pressure to the fuel cell operation. The fuel pressure could bechosen in order to modify and/or control the water balance across thefuel cell. The operation point of the fuel cell may be selected byevaluating operating variables such as but not limited to temperature,pressure, gas composition, reactant utilizations, water balance, andcurrent density as well as other factors such as impurities and celllife that influence the ideal cell potential and the magnitude of thevoltage losses. In prior art systems, there is often a ‘time delay’between a change in load applied to the system, and the systemresponding to the change in applied load. Method embodiments of theinvention described herein eliminate the time delay and problemsresulting from the time delay because the method embodiments of theinvention rely upon a constant application of an internal fuel feedpressure to the fuel enclosure. For some embodiments, the internal fuelfeed pressure is pre-selected. No other fuel feed control is required.For some embodiments, the only means of fuel feed control is a pressureregulator. For some embodiments, instead of being pre-selected, the fuelfeed pressure is controlled through the pressure regulator and can bemodified based on any number of desired parameters, for example,environmental conditions, power demand, and/or quantity of fuel.

Because the fuel is provided to the fuel cell system in excess of thereaction demand, the fuel control allows for more flexible operationwithout dynamic control. Control of pressure rather than flow rateallows for improved, stabilized, fuel supply control. Additionally,control of fuel supply by use of pressure control rather than flow ratesimplifies fuel supply to a fuel cell or fuel cell stack because thepressure control is independent of load demands. While feedback controlhas been described for regulating pressure of the fuel to the fuel cell,assembly, it is understood that other types of control may be suitablefor specific types of applications. The fuel cell system also includes,for some embodiments, sealants, such as those shown at 126 in FIG. 5,that prevent loss of fuel from the fuel cell system. The fuel cellsystem may also include a positive electrical connector and negativeelectrical connectors.

Examples of application of method embodiments described are presentedherein. The Examples are presented to better describe the methodembodiments and not to limit them.

The area of each individual fuel cell in the planar array can be in therange of 0.00000001 cm² to 1000 cm².

EXAMPLE 1

The test results shown in FIG. 2 illustrate the difficulty in attainingextended operating times and stable performance at the same time asachieving high fuel utilization. FIG. 2 illustrates the operation of afuel cell system in both dead-ended and open-ended modes with differentfuel utilizations. In all four tests the fuel cell cathodes were merelyexposed to ambient air for the supply of oxidant; pure unhumidifiedhydrogen from a compressed gas cylinder was directed to the anodes. Thefuel cell studied was 10-cell assembly operated at 200 mA/cm²

Curve A shows the voltage against operating time when the fuel cellsystem was operated open-ended at less than 1 psig of hydrogen pressure(i.e. with hydrogen flowing past the anodes and exiting via an outlet).The flow rate was such that the fuel utilization was about 90%—in otherwords, most of the hydrogen supplied to the anode was consumed. At thishigh fuel utilization (and correspondingly low hydrogen flow rate) thefuel cell performance decayed dramatically after only about 5 minutes.

Curve B, the flat curve that goes all the way across the graph, showsthe same type of open-ended operation but at a flow rate such that thefuel utilization was only about 40%—in other words substantial excesshydrogen was supplied to the anode and exited the fuel cell system via afuel outlet. Under these conditions the fuel cell exhibited stableperformance for more than an hour (at which time the test wasintentionally stopped).

Curve C, the curve with 2 dips, shows the same fuel cell systemoperating dead-ended on hydrogen (the fuel outlet was closed) with ahydrogen utilization of about 90%. The pressure differential from anodeto cathode was initially about 0.25 psig. As the voltage began todecline the anode-to-cathode pressure differential was increased toabout 2.85 psig. Despite the increase in fuel pressure the fuel cellvoltage decayed dramatically after only about 20 minutes. Upon openingthe fuel outlet valve briefly and allowing a small amount of hydrogen tovent from the fuel cell system, the fuel cell voltage was restored for ashort period but decayed again within minutes with the outlet closed,despite the pressure differential of about 2.85 psig.

EXAMPLE 2

In this Example the same 10-cell assembly as in Example 1 was operateddead-ended on hydrogen with a much higher anode-to-cathode pressuredifferential—this time about 24 psig. Again the cathodes were merelyexposed to ambient air for the supply of oxidant. A graph of voltageagainst operating time during this dead-ended operation is shown in FIG.3. The graph shows that the fuel cell operated at a voltage of betweenabout 7.0 and 7.5 volts for over 25,000 seconds (almost 7 hours), atwhich time the test was intentionally stopped. This illustrates theadvantage of using a high pressure differential in a passive,air-breathing fuel cell system that is dead-ended on the fuel side. Forthis particular type and size of fuel cell system under these operatingconditions a 2.85 psig differential was not sufficient (as shown byExample 1), but a 24 psig pressured differential gave a significantimprovement in operating time.

EXAMPLE 3

In this Example the present method was tested using a fuel cellarchitecture different from that utilized in Examples 1 and 2 (asdescribed in ELECTROCHEMICAL FUEL CELLS HAVING CURRENT-CARRYINGSTRUCTURES UNDERLYING REACTION LAYERS), voltage versus operating timewas measured at 200 mA/cm², as shown in FIG. 4. The surroundingtemperature was also monitored, and is shown in the graph. Again thefuel cell cathodes were merely exposed to ambient air for the supply ofoxidant. Hydrogen was supplied to the anodes, which were dead-ended, ata pressure of about 5 psig. The data show that the stack operated withina voltage range of 4 to 8 Volts for about 1900 hours (over 10 weeks).The hydrogen remained dead-ended throughout—no venting or purging. Theambient temperature ranged from about 20 to 35° C.

Thus, for this particular fuel cell architecture under these operatingconditions an anode-to-cathode pressure differential of 5 psig wassufficient to allow stable, dead-ended operation on hydrogen for anextended period of time.

For some embodiments, fuel cell assemblies employed in the methodsdescribed herein are integrated into a housing of an electricallypowered device. The integration of fuel cells with the housing of anelectrically powered device provides the opportunity for portions of thecathode region of the fuel cells to form a portion of the exterior ofthe device enclosure. This can save space. In some embodiments, thecathodes are exposed to the surrounding environment, while the anodesand fuel plenum are located on an inner surface of the fuel cell system.It is contemplated that the method and fuel cell embodiments describedherein can be incorporated into an electronic device. Such electronicdevice could be, for example, the following: a cellular phone, a PDA, asatellite phone, a laptop computer, a portable DVD player, portable CDplayer, a portable personal care device, portable stereo, a portabletelevisions, a radar, a radio transmitter, radar detectors, a laptopcomputer, and combinations thereof.

In the description of some embodiments of the invention, reference hasbeen made to the accompanying drawings which form a part hereof, and inwhich are shown, by way of illustration, specific embodiments of theinvention which may be practiced. In the drawings, like numeralsdescribe substantially similar components throughout the several views.These embodiments are described in sufficient detail to enable thoseskilled in the art to practice the invention. Other embodiments may beutilized and structural, logical, and electrical changes may be madewithout departing from the scope of the invention. The followingdetailed description is not to be taken in a limiting sense, and thescope of the invention is defined only by the appended claims, alongwith the full scope of equivalents to which such claims are entitled.

What is claimed is:
 1. A method for operating a fuel cell system with apassive reactant supply, providing a fuel cell system, the fuel cellsystem comprising a first fuel cell layer that includes a plurality ofanodes, a plurality of cathodes, and an ion-conducting electrolyte, theplurality of anodes and plurality of cathodes arranged in a planar arraywith the plurality of anodes arranged adjacently on a first side of thefirst fuel cell layer and the plurality of cathodes arranged adjacentlyon a second side of the first fuel cell layer opposite the first side,wherein an area of each individual fuel cell in the planar array is inthe range of 0.00000001 cm² to 1000 cm², the fuel cell system furthercomprising a closed fuel supply fluidly in contact with the plurality ofanodes; exposing the cathodes of the planar array to ambient air; andsupplying a fuel to the anodes of the planar array at a pressure greaterthan that of the ambient air; wherein a fuel utilization of the systemis greater than 75%.
 2. The method of claim 1 wherein the fuel ishydrogen.
 3. The method of claim 1 wherein the fuel utilization isgreater than 90%.
 4. The method of claim 1 comprising controlling a fuelsupply pressure using at least one pressure regulator.
 5. The method ofclaim 1 wherein the fuel supply comprises a hydrogen storage materialselected from the group consisting of: a metal hydride, a compositemetal hydride, carbon-graphite nanofibers, compressed hydrogen gas, anda chemical hydride.
 6. The method of claim 1 comprising supplying thefuel to the plurality of anodes at a substantially constant pressure. 7.The method of claim 1 comprising providing the fuel to the plurality ofanodes at a pressure that is independent of a power demand of the fuelcell system.
 8. The method of claim 1 wherein supplying a fuel comprisesproviding the fuel without providing components of pressure regulationbetween the closed fuel supply and the plurality of anodes.
 9. Themethod of claim 1, comprising providing the fuel supplied to the anodesat a pressure that is dependent on a power demand of the fuel cellsystem.
 10. The method of claim 1, wherein supplying a fuel comprisesproviding the fuel through one or more pressure control componentsdisposed between the closed fuel supply and the plurality of anodes. 11.The method of claim 1, wherein supplying a fuel comprises providing thefuel to the anodes from a metal hydride.
 12. The method of claim 1wherein supplying a fuel comprises providing the fuel in anon-humidified form.
 13. The method of claim 1 wherein supplying a fuelcomprises providing the fuel to each of the plurality of anodes inparallel.
 14. The method of claim 1 wherein supplying a fuel comprisesproviding the fuel to at least a portion of the plurality of anodes inseries.
 15. The method of claim 1 wherein the ion-conducting electrolytecomprises a proton exchange membrane.
 16. The method of claim 1 whereinthe ion-conductive electrolyte comprises a polymeric perfluorosulfonicacid.
 17. The method of claim 1 wherein an ion-conductivity of theion-conductive electrolyte is dependent on a hydration level of theion-conductive electrolyte.
 18. The method of claim 1 wherein athickness of the ion-conductive electrolyte is in the range of 1 micronto 100 microns.
 19. The method of claim 1, wherein the fuel cell systemis integrated into a housing of one or more of a portable electricalpower source, cellular phone, PDA, satellite phone, laptop computer,portable DVD player, portable CD player, portable personal care device,portable stereo, portable television, radio transmitter, radartransmitter, radar detector, laptop computer, any portable electronicdevice, any portable communication device and combinations thereof. 20.The method of claim 1, wherein the fuel pressure is modulated.
 21. Themethod of claim 1, wherein fuel is not released from the fuel cellsystem by purging or venting the fuel from the anodes.
 22. A method foroperating a fuel cell system, the method comprising: providing a fuelcell system, the fuel cell system comprising at least one planar fuelcell layer and a passive reactant supply, the fuel cell system furthercomprising at least two anodes, at least two cathodes, with anion-conductive electrolyte disposed between each anode and cathode andthe at least two anodes arranged adjacently on a first side of theplanar fuel cell layer and the at least two cathodes arranged adjacentlyon a second side of the planar fuel cell layer opposite the first side,wherein an area of each individual fuel cell in the planar array is inthe range of 0.00000001 cm² to 1000 cm², the fuel cell system furthercomprising a fluidly-coupled fuel supply; pressurizing the fuel supplyto a pressure greater than an ambient pressure; and exposing the cathodeof the at least one planar fuel cell to ambient air; wherein a fuelutilization of the system is greater than 75%.
 23. The method accordingto claim 22 wherein the fuel supply comprises a metal hydride.
 24. Themethod of claim 22, further comprising a closed fuel plenum sealablyenclosing the anodes, wherein the fuel plenum is in fluid communicationwith the fuel supply, wherein the method further comprises supplying afuel to the anodes of the planar fuel cell array via the fuel plenum ata pressure greater than that of the ambient air.
 25. The method of claim22, further comprising supplying fuel to the at least two anodes at apressure greater than that of the ambient air, wherein the pressure isselected so that during steady state operation, the at least one planarfuel cell achieves water balance such that the amount of water producedby the fuel cell system is substantially equal to the amount of waterdischarged from the fuel cell system.
 26. The method of claim 22,further comprising supplying fuel to the at least two anodes of theplanar fuel cell array at a pressure greater than that of the ambientair, wherein the pressure is effective in reducing nitrogen diffusionfrom the cathodes to the anodes of the planar fuel cell array and formaintaining adequate hydration of the electrolyte.
 27. A method ofoperating a planar array of fuel cells, the method comprising: providinga planar array of fuel cells comprising at least two unit fuel cellsarranged on a single fuel cell layer, wherein an area of each individualfuel cell in the planar array is in the range of 0.00000001 cm² to 1000cm²; providing an enclosure proximate to anode portions of the planararray that sealably encloses the anode portions of the planar array;admitting a fuel into the enclosure and retaining the fuel within theenclosure while maintaining a predetermined fuel pressure; and exposingcathode portions of the planar array to ambient air; wherein thepredetermined fuel pressure is greater than a pressure of the ambientair; wherein a fuel utilization of the planar array is greater than 75%.28. The method of claim 27, wherein exposing cathode portions of theplanar array to ambient air comprises supplying the ambient air to thecathode substantially by diffusion.
 29. The method of claim 27,comprising disposing one or more protective layers on the exposedcathode portions, including at least one of a carbon layer and ahydrophobic layer.
 30. The method of claim 27, comprising positioning aporous protective cover proximate the exposed cathode portions,including at least one of a hydrophobic sheet, an activated carbonfilter, and a screen or mesh.
 31. The method of claim 27, furthercomprising: coupling the planar array of fuel cells to an electricalload; and transferring electrical energy from the planar array of fuelcells to an electrical load.